The present invention relates to a steering apparatus controller for an electric power steering apparatus that applies assist torque to a steering system of a vehicle by a motor.
A typical electric power steering apparatus uses rotational force of an electric motor to assist steering of a steering wheel. In such an electric power steering apparatus, an electric motor applies assist torque that corresponds to the actual steering torque to a steering mechanism when a driver steers the steering wheel.
The electric power steering apparatus includes a steering apparatus controller for controlling the applied voltage (motor drive voltage) to the electric motor. The steering apparatus controller has a motor driving device for driving the electric motor and a central processing unit (CPU) for sending command signals to the motor driving device in accordance with the motor drive voltage.
The steering apparatus controller computes a motor drive current value required for the electric motor to generate the desired assist torque. The steering apparatus controller detects the current (actual current) that actually flows through the electric motor and executes a feedback control to determine the motor drive voltage with reference to the detected value of the actual current.
In the feedback control, if, for example, the actual current detection value decreases for some reason when the steering torque is constant, the steering apparatus controller increases the motor drive voltage to prevent the assist torque from dropping. The feedback control permits the appropriate assist torque to be reliably obtained.
An amplifier that uses, for example, an operational amplifier, is generally located in the device for detecting the actual current. The amplifier amplifies a minute signal sent from the electric motor by a predetermined gain at the operational amplifier and sends the amplified signal to the CPU. Power required for the operational amplifier to amplify the minute signal is supplied to the operational amplifier from the power source. The gain of the amplifier is maintained at a predetermined value by maintaining the voltage supplied to the operational amplifier, or the sensor drive voltage, at a predetermined level due to the structural feature of the operational amplifier.
However, if the sensor drive voltage becomes less than the predetermined level for some reason, it becomes difficult to maintain the gain at the predetermined value and the gain might decrease. In this case, a phenomenon as shown in FIG. 6 is induced.
FIG. 6 illustrates the relationship among the steering torque 101, the sensor drive voltage 102, the actual current detection value 103, the motor drive voltage 104, and the actual current value 105, which actually flows through the motor. In FIG. 6, the steering torque 101 is constant with respect to the temporal transition.
As shown in FIG. 6, if the sensor drive voltage 102 starts to decrease for some reason and become less than a predetermined voltage Vp, the actual current detection value 103 decreases (t10 to t20) according to the decrease of the gain of the operational amplifier although the steering torque is not changed. The voltage Vp is a minimum voltage value required for amplifying the signal with the normal gain at the operational amplifier.
When the actual current detection value 103 decreases, the steering apparatus controller increases the motor drive voltage 104 to prevent the assist torque from dropping. This increases the actual current value 105.
In this case, since the steering torque is not actually changed, the increase of the current value of the actual current causes the electric motor to generate excessive assist torque. Since the excessive assist torque turns the steering wheel excessively, the steering torque, for example, might be applied to the torque sensor, which detects the steering torque, in the direction opposite to the direction that the driver has been applying the steering torque. If the steering torque is applied to the torque sensor in the opposite direction, the steering apparatus controller reverses the rotational direction of the electric motor. If this is repeated, the steering mechanism causes vibration, which deteriorates the steering feeling.
The objective of the present invention is to provide a steering apparatus controller for an electric power steering apparatus that prevents deterioration of the steering feeling due to vibration of a steering mechanism.
To achieve the above objective, the present invention provides a steering apparatus controller for an electric power steering apparatus. The steering apparatus controller executes a feedback control of an applied voltage to an electric motor. The electric motor generates assist torque for assisting steering of a steering wheel. The assist torque is based on a detection result of an actual current that flows through the electric motor. The steering apparatus controller includes a sensor and a voltage controller. The sensor detects the actual current. When the sensor detects an abnormal decrease of a drive voltage of the sensor, the voltage controller maintains the applied voltage to the electric motor to a drive voltage at a predetermined time before the drive voltage abnormally decreased.
The present invention also provides a controlling method for an electric power steering apparatus. A steering apparatus controller executes a feedback control of an applied voltage to an electric motor. The electric motor generates assist torque for assisting steering of a steering wheel based on a detection result of an actual current that flows through the electric motor. The detected actual current is detected by a sensor. The method includes: detecting an abnormal decrease of a drive voltage of the sensor; and when detecting an abnormal decrease of the drive voltage of the sensor, maintaining the applied drive voltage to the electric motor to a drive voltage at a predetermined time before the drive voltage abnormally decreased.
Other aspects and advantages of the invention will become apparent from the following description, taken in conjunction with the accompanying drawings, illustrating by way of example the principles of the invention.